Masterclass: Advanced Dynamics Processing Tricks

We often cover the basics of dynamics processing in these pages, focusing
mainly on simple applications of compression, limiting, gating, and expansion,
but advanced processing functions can be achieved using dynamics processors,
through the use of techniques known as sidechaining and keying. This month we’ll
explain the concepts behind key inputs and sidechains, and explore applications
for their uses. Most of these ideas apply to hardware and software processing,
though the execution varies somewhat. While you’re reading through this, please
remember that the key input or sidechain to a dynamics processor is separate from
the audio path.

Example of automated key operation, using MOTU Digital Performer.

Watching the Detectors All dynamics
processors contain a control circuit, often referred
to as a detector. This is the hardware circuit
(or software emulation thereof ) that does the
thinking for the device. The detector analyzes
the audio signal and—based upon the way you
configure controls—applies the requisite gain
control. [Editor’s note: the sound of a compressor
is all about the type of detector circuitry the
designer has employed. Optical, Vari-Mu, and
VCA compressors possess very different gain
control characteristics, resulting in distinct sonic
characters—all of which are eminently useful.
That’s a discussion for another time.]

Under most circumstances, the detector
is listening to the same audio signal that is
being processed. This sounds deceptively
simple: After all, why wouldn’t the detector
be controlled by the same audio that it’s
processing? The answer is, a lot of advanced
dynamic effects can be created when the
detector reacts to a signal other than the
audio signal it is processing. To accomplish
this, a hardware compressor must have
a separate key input or a sidechain path
consisting of a dedicated input and output.
You can route anything you want to this input
and use it to take control over the detection
process. For example, you can route a kick
drum to the detector and let the kick drum
govern gain reduction on a vocal; see the
block diagrams shown in Figures 1A and
1B. Figure 1A (right) shows audio in the
compressor being split and routed to the
audio path and the detector. Figure 1B shows a vocal in the audio path, but a
kick drum is routed to the detector’s key
input. Every time the kick drum is hit, the
compressor acts on the vocal.

Fig. 1A. Audio
sent through the
compressor is
split and routed
to the audio path
and the detector.

The beauty of using plug-ins is that many
of them feature a key input, even if that
plug-in is emulating a hardware device that
did not have a key input. For example, the
original Universal Audio 1176 limiter did not
have a key input, but the Bomb Factory BF76
1176 emulation does, letting you accomplish
processing that was impossible with the
original hardware unit.

Many plug-ins feature a key input, even if that plug-in is emulating a hardware device
that did not have a key input. For example, the original Universal Audio 1176 limiter did not
have a key input, but the Bomb Factory BF76 1176 emulation does (see upper left corner),
letting you accomplish processing that was impossible with the original hardware unit.

Key Control Enabling key operation
simply requires feeding any signal you’d like
into the key input. Let’s take as an example
the commercial “donut”: Radio and TV
commercials typically start with a few seconds
of music, and then a voiceover comes in to
deliver the sales pitch. In the olden days, an
engineer would manually lower the music
when the voiceover started so that the music
would not compete with the message. When
the voiceover ended, the music would briefly
return to the original level before fading out.

This process can be automated by
using the voiceover to control the gain of a
compressor inserted on the music track. The
routing for this process is demonstrated in
the screen shot of Digital Performer’s mixer
window. In this window,
the track on the left (highlighted red) is the
voiceover. The track on the right (highlighted
blue) is the music.

MOTU’s dynamics processor is inserted
on the music track and is set to Compressor.
The drop-down menu for the Control Signal
is open. You’ll see Input at the top of the menu
and a list of buses underneath. When the
Control Signal is set to Input, the dynamics
processor behaves as you’d expect: It acts
based upon what the music is doing—i.e. when
the music track gets louder, compression
increases. However we have set the Control
Signal to Bus 1.

The Voiceover track has a send on Bus 1;
the send knob is turned up and the send is set
Prefader so that the key signal is independent
of the vocal fader. This routing enables
the compression on the music track to be
“triggered” or “keyed” from the voice track.
When the voice starts, it feeds audio to the
Control input of the dynamics plug-in, causing
the music track to be compressed.

When the voiceover ends, the signal at
the Control input ceases and the music track
returns to normal volume. This process is
known as a ducker. Note that once the Control Signal is set to Bus 1, the compressor will not
function unless there is a signal present on Bus
1. A similar technique can be used in a karaoke
situation or for a restaurant paging systems
in which the announcer’s voice ducks the
background music.

Fig. 1B. In this diagram,
the kick drum is the
control signal, keying
compression on vocals.

There are other uses for a ducker, one of
which is a compression trick from back in the metal days. If you have a song with distorted
rhythm guitars and you want to keep them “in
yer face,” use the lead vocal to key compression
on the guitar tracks. Every time the vocal
enters, the guitars are reduced in level. You’ll
need to set the parameters of the compressor
so that the effect is subtle: Try medium to fast
attack and quick release times so that as soon as the voice starts, the guitars duck and as
soon as the voice ends the guitars come back
to normal. Start with a ratio of 2:1 or 3:1; you’ll
need to play around with the threshold to
achieve just a few dB of compression so that
the effect is not obvious. The nice thing about
doing this in a DAW (as opposed to the old
analog days) is that you can easily route the
vocal to key compressors on multiple guitar
tracks without the hassle of physically splitting
the signal and connecting a lot of patch
cables etc. Simply set the key inputs of the compressors to the same bus as the vocal send.

I use the same technique all the time with
delay effects on lead vocal, live and in the
studio. Ducking the delay while the vocalist is
singing keeps the vocal in front and maintains
clarity. When the vocal ends, the delay comes
up, making it more audible.

Fig. 2. Here, the vocal is
fed into the compressor,
and an EQ is patched
into the sidechain.

Filter Your Mouth Sidechaining is a similar
concept, but involves sending the detector
signal out to an external processor, altering
it, and then returning it back to the detector
input. Patching an EQ into the sidechain is
very common, and can be used to create a
de-esser. A de-esser is actually a compressor
that has been made sensitive or “tuned” to sibilant frequencies. This process is achieved
by applying an EQ to the signal before it
returns back to the detector. In the block
diagram shown in Figure 2, the vocal is fed
into the compressor, and an EQ is patched into
the sidechain. (Note that the compressor’s
sidechain switch must be engaged or the EQ
is not applied.) I usually dump out all of the
frequencies below around 3 kHz and apply a
severe boost in the upper mids (anywhere from
4 to 8 kHz, depending upon the singer. You’ll
have to experiment), making the compressor
very sensitive to sibilance. If you set the
threshold, attack and ratio controls carefully,
the compressor will act on “s” sounds but
other sounds will not trigger compression.

Sidechain filtering can also be used to avoid
excessive compression due to a high content
of low frequencies in a mix. Low frequency
sounds carry a lot of energy in a mix and
they can trigger compression that causes
audible side effects. For example, you might
notice that every time the kick drum hits,
the lead vocal gets sucked down. The cure is
to remove some of the low frequencies from
the compressor’s sidechain (say, everything
below 200 Hz). Filtering the bottom end stops
the compressor from kicking in every time a
kick drum is hit, but remember—you have not
filtered the audio path.

“Smarter” hardware gates such as the Drawmer DS404 feature front panel controls for low- and high-frequency filters. Using both filters
simultaneously allows you to build a bandpass filter that removes some of the sounds that you do not want opening the gate.

Keys to the Kingdom Similar concepts can
be applied to expander/gates. As with their
compression counterparts, hardware gates will
feature a key or trigger input jack, usually with an
associated switch on the front panel. Plug-ins will
offer a key on/off button along with a drop-down
menu that allows you to choose a key input. This
could be a physical input on your audio interface
but more likely will be a bus that will receive a
signal from elsewhere in the session.

Think of a gate as a door that opens or
closes based on the strength of the signal at
the doorway. If the signal is strong enough the
door is pushed open, but the key or trigger
input of a gate is like an electric door latch.
That latch opens or closes based on a remote
signal—regardless of the strength of the audio
that is attempting to pass through the doorway.
A gate that provides a key input allows you to
use a secondary sound to open the “doorway.”
Let’s say you insert a gate on a synth bass track
but route the kick drum to the gate’s key input.
The synth bass itself will not open and close
the gate—the kick drum will control the gate,
allowing the synth to be heard or not. This can
be used for some interesting effects (usually
in the studio) where the synth is musically
tightened up to the kick drum hits. In fact
such a technique could be used to change the
rhythm of the synth bass so that it precisely
matches that of the kick drum. Swap the synth
bass for a test tone generator tuned between
50 and 80 Hz, and you’ll have a TR808 kick
sound. A similar technique can be used to
tighten up “gang” vocals by keying them from
the lead vocal track.

Live engineers on major tours have been
known to use contact pickups or triggers on
each drum to key that drum’s gate instead
of using the signal from the microphone to open the gate. Let’s say you have a mic on a
snare drum and you are attempting to gate
the mic. Depending upon placement of that
microphone and the player’s touch on the kit,
audio from other components of the kit such
as toms, cymbals and kick drum may leak into
the microphone, causing false triggers that
open the gate even when the snare is not hit.
If we add a contact pickup or trigger to the
snare drum and route it to the gate’s key input,
the gate opens and closes much more reliably
because the trigger is in physical contact with
the drum and is much less subject to leakage
(though some contact pickups may be sensitive
to vibration). This also means that softer hits
on the snare can open the gate reliably so that
grace note-style hits are not muted. As a bonus,
we could split the signal from the trigger or
contact pickup and send it to the trigger input
of a drum module—allowing us to layer a
sampled snare with the real snare or possibly
record the performance as MIDI data.

Gates that have a sidechain (or a sidechain
filter) can be especially useful because they let
you filter unwanted sound from the sidechain—preventing them from opening the gate and
effectively making the processor more sensitive
to the sounds you do want opening the gate. A
great application for a sidechain filter would be
when you have significant kick drum leakage
into a snare drum track, and the kick drum
is causing the snare gate to open. By filtering
the low frequencies from the sidechain, the
gate will “hear” less of the kick drum and
respond more to the snare. Remember that
this filter applies to the sidechain and not
to the main audio path. Most plug-in gates include a sidechain filter (Waves’ Renaissance
Channel for example), so take advantage of it.
“Smarter” hardware gates such as the Drawmer
DS404 feature front panel controls for low- and
high-frequency filters. Using both filters
simultaneously allows you to build a bandpass
filter that removes some of the sounds you do
not want opening the gate. You’ll also typically
find a sidechain “listen” switch that enables you
to temporarily hear the filtered sidechain signal
via the audio outputs. This is extremely useful
in tuning the filters to pass only the signal you
are trying to gate.

Take a Look Ahead One feature that is
exclusive to software gates (and compressors)
is the look-ahead function. Here’s how it
works: A gate or compressor can only act
upon a signal when it reaches the input of the
processor. In some cases, a transient such as
a snare hit might fly right past the gate before
the gate can respond—so that hit will be
muted. Look-ahead allows the gate’s sidechain
to hear the signal before it hits the gate’s audio
input (did someone say “cheating”?), allowing
it to open just before the sound reaches the
gate’s audio input. MOTU’s MasterWorks Gate
is great for this because you can set the look-ahead
in milliseconds from 0 to 20. I find that
setting it to 1 or 2 milliseconds is just enough
that the gate can be set tightly enough to cut
leakage but not chop off the leading edge of
a snare or tom hit. A similar function on a
compressor plug-in can help catch transients
that are faster than the comp’s attack time.

It’s worth mentioning that if you are using
hardware compression or gating, be careful
about processing during recording. If a gate
inadvertently removes audio that you wanted
(that grace note on a snare, for example), it
cannot be recovered. This is much less of an
issue in DAWs because plug-in effects are
almost always non-destructive—in other
words, they are applied in the monitor path
and can be removed without harm to the
original audio file.

Steve La Cerra is an independent audio
engineer based in New York. In addition to
being an Electronic Musician contributor,
he mixes front-of-house for Blue Öyster Cult
and teaches audio at Mercy College Dobbs
Ferry campus.